


Make me an adult

by Egirl1423, Yaminisa



Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell, Alternate Universe - Underswap, Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), First Time, I'm Bad At Summaries, M/M, Underfell Sans, Underswap Sans
Language: English
Status: Completed
Published: 2016-07-16
Updated: 2016-12-11
Packaged: 2018-07-24 07:48:56
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 3,939
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7500012
Author URL: https://archiveofourown.org/users/Egirl1423/pseuds/Egirl1423, https://archiveofourown.org/users/Yaminisa/pseuds/Yaminisa
Summary: <blockquote class="userstuff">
              <p>Blueberry is tired of everyone always treating him like a child, and Red is just tired, and wants to sleep. He finds a way that will finally make people treat him like an adult, but he will need Red’s help.</p>
            </blockquote>





	1. Chapter 1

**Author's Note:**

> Us!Sans is Blueberry, Blue or just Berry while UF!Sans is Red. Hope you enjoy.

“It’s always the same thing. They always say that I’m too young, and I’m tired of it!” Blueberry complained.

 

Red groaned into the pillow he had put on his face hours ago. For two or three hours now, Blueberry been talking nonstop about something, he didn’t even pretend to listen anymore.

He liked the little guy, he really did, after all he was the reason why his bro got along with him. But right now, all he wanted to do was sleep.

 

“Red, do you think I’m too childish?” Blueberry asked, looking at him with his big blue innocent eyes.

 

“yes” he answered, still not paying attention.

Blueberry pouted, and Red knew that he hadn’t given a satisfying answer, maybe he  _ should  _ pay attention.

 

“Red, do you know if there is anything that only grown ups do?” Red opened his eyes, to see  Blueberry, merely inches from his face, starry hoping eyes looking at him.

 

“i-i don’t know, usually everything 18+ are about alcohol, drugs or sex…” Red answered, doing his best to censor himself.

 

“Drugs and alcohol are bad, but what’s sex?”

Red choked at the question. Shit, he didn’t want to have to give Blue the birds and the bees talk, that should be Swaps job, not his.

 

“Red, what is it?” the innocent one insisted.

 

I’ll have to improvise, Swap would never let me hear the end of it if I ever ruined the little ones innocence. Guess I’ll stick to the basics.

 

“ok, so when you really like someone, for example your girlfriend, after a while you might want to take your relationship to the next step, so you usually have sex.” He summarized, in hopes that Blueberry would be satisfied with the answer.

 

“But what if I’m not attracted to girl?” Blue asked.

 

Red was surprised, since this version of himself was supposed to be closer to his brother than he was to his. That would mean that Swap might have the same sexual orientation as Berry, this could be fun.

“that only means that you’re gay.” He said.

 

“Can two guys have sex together?” Blue’s eyes shone with curiosity.

 

“it isn’t the same as girl and boy sex, but yeah.” This conversation wasn’t going as bad as he thought it would be.

 

“Red, are you gay too?”

Forget it, it didn’t go as planned. 

 

He sighed before answering.

“yes, i am.” The little skeleton smiled at his answer.

 

“That means you could be my boyfriend!” Blue almost yelled, sounding so excited by this.

 

What. The. Hell?! All he just wanted was for Blueberry to shut up and sleep, and here they were discussing relationships and sex!

“well, yeah i guess i could-”

 

“Then Red, do you want to go out with me?” interrupted Berry.

 

He didn’t know how to answer. He did have feelings toward Berry, but this was a little out of the blue. He look at Blueberry, who was looking at him with hopeful eyes, a heavy blush on his face.

 

“i know we haven’t gone on a date together, but i really didn’t know how to ask you out. all i can say is that i love you.” Red confessed to Blueberry.     

He was being honest, it was one of the things he liked about himself. Honesty didn’t exist in his world anymore.

“so yes, Blueberry, i want to be your boyfriend.” He said.

He smiled at his new boyfriend, who was rolling around in happiness, how cute.

Blue hugged him close, and Red tensed at first, he was not used to cuddling,  guess I’ll have to get used to it soon, not that he minded.

Red hugged the little one closer and closed his eyes, his body quickly relaxing.

Just as he was about to fall asleep, Blue spoke up.

 

“Red, can I kiss you?” Blue asked.

Normally he would have let the now soft and small voice drift him to sleep if it wasn't for the tempting offer.

 

“yeah sure.” Red answered.  He lay still, and let the smaller do the work, barely stopping himself from laughing when he only felt a chaste kiss on his teeth. Blueberry really was innocent.

Red parted his teeth and licked him to see how he would react. Blue pulled away, in surprise.

 

“Red…there was something wet?” He said confused.

Red opened his mouth, exposing his tongue for Blue to examine.

 

“What is this?” He innocently asked.

 

“a tongue.” Red simply answered.

 

“Why is it there?”

 

“i summoned it, for kissing.” he smirked, and moved his hands to caress Blue’s ribs.

 

Blueberry concentrated hard, soon forming a tongue himself. He kissed Red again, who kissed back, caressing Blue’s ribs a little bit more roughly.

Red used the moment when Blueberry let out a little moan, to let his tongue wander in the other’s mouth.

There wasn’t much of a fight for dominance, but Blueberry would learn by experience. He pulled back when he felt like air was needed.

Blueberry’s pants and moans sounded great, and he bet it would sound even better when he was yelling and moaning his name. He mentally slapped himself for thinking about that, his boyfriend was too innocent for that, hell they had only been a couple for five minutes. Blue deserved better than sex the first night.

“Red.” Blue panted, interrupting him from his lewd thought.

 

“what’s wrong babe” he asked.

 

“It feels really weird, between my legs, what’s happening to me?” He unconsciously rubbed his legs together.

 

“your body is getting aroused, which means that it wants to have sex.”

Berry nodded in understanding.

 

“And could we? Have sex I mean?”

 

If he couldn’t say no to the offer of kissing, how could he say no the even more tempting offer of taking Blueberry’s virginity.

He gently pushed the other onto his back and got on top of him. He took off his shirt, and unbuttoned Blue’s pajama top, admiring the little baby blue soul that was softly shining.

He knew what he could do with it, but he would keep that for another night. He didn’t want to have to explain soul sex.

He played with the ribs, it seemed that Blue's ribs were as sensitive as his own. The little one let out cute whimpering moan, not even trying to contain or stop himself.

It didn’t take long before he saw a soft glow coming from Blue’s pants. His hands went to the pants, but Blueberry’s hands were on his in an instant.

 

“Papy said to never let anyone take my pants off.” Berry said looking away.

 

“well, if you still want to have sex, we’ll need those off, but you could always take them off yourself.” He proposed.

Blueberry seemed to agree with it, because soon he heard the sound of clothes hitting the floor. Berry tried to cover himself with his hands, and Red found the gesture really cute. Red took off his own pants before pushing Blue’s hands aside, letting him finally see what his lover had conjured.

A little blue cunt was glowing between the two femurs. It was dripping wet and would no doubt be tight.

 

“guess i wasn’t wrong when i called you a pussy.” He chuckled, and Blueberry’s protest was forgotten, when Red pushed a single digit inside of him.

A lustful smile made its way onto Red’s face, as he felt Blue’s pussy clenched around him. It was tight, wet and hot, he couldn't wait to be deep inside of it.

He prepared Blueberry as best as he could, keeping him on the edge but not letting him climax. Red slowly pushed himself inside, groaning as he did. He looked at his lover's face, blue tears welled at the corner of his eyes sockets, but his eyes were heart-shaped and his face expressed pure bliss. He push himself all the way, then stopped so Berry could adjust.

 

“fuck, your cunt is so good Blue.” he groaned.

 

“Red, langu-AHHHH~”

Red chuckled, proud of what he just did, as he continue to trust.

 

“what’s wrong kitten, cat got your tongue?” he said between his own groans.

He might act like a smart ass, but he was just as lost in the bliss as Blueberry was.

He started thrusting faster, Blue’s high-pitched moans encouraging him.

 

“hey blueberry, how do you feel about a creampie?” he asked between grunts.

 

The little one tried to answer but a hard thrust stopped him each time.

“i feel like now would be a great time to make one.” he chuckled.

 

He moved faster and harder, feeling like he was getting close. One of his hands when to Berry’s blue clit, rubbing it in pace with the thrusts.

He felt his lover tense under him as he moaned louder, coming for the first time in his life. Not soon after, Red came in him, pulling out to see his cum drip down Blue’s slit.

He let himself fall next to Blueberry, and wrapped his arms around him.

 

“Hey Red, now that we have had sex, do you think that people will stop thinking I’m childish?” Blue asked when he finally came back to reality.

 

“yeah, no doubt, now let’s get some sleep, I’m fucking tired.” he said, closing his eyes.

 

“Red, language, and I love you.” Blue hugged him close.

  
“love you too.” Red whispered before finally falling asleep.


	2. Chapter 2

**Notes for the Chapter:**

> I'm sorry for the wait...

 

Red woke up to find that Blueberry was missing. The little one was probably downstairs cooking breakfast tacos.He got dressed and went to the kitchen, where he found the three skeletons.His brother was reading the newspaper, a warm coffee mug in front of him.

Swap’s head was on the table, his hood up, and if he paid enough attention, he could hear soft snoring.

Blueberry placed the plates on the table and intertwined his fingers with Red’s under the table. They start eating and Swap finally looked up at them.

He frowned as he looked at his brother and his copy. The scene was weird in a funny way but something was wrong. They both had a happy smile on their face, both eating at the same time, and taking a bites in sync. But they only ate with one hand, a thing his brother never did before, plus their hands were reversed. He had a guess as to why, but he wasn’t sure how he felt about it.

 

“hey bro, you seem really happy this morning, more than usual.” Swap said.

 

His brother's smile brightened, and he looked at Red tenderly.

 

“Well, I talked with Red last night, and now we are boyfriends” Blue answered.

 

Red moved their hands onto Blue’s thigh, caressing his femurs softly through the fabric. Might as well have some fun before being dusted by Swap.

As soon as he know his brother wasn’t a virgin anymore, he would put two and two together and dust him. He understood, after all, he would do the same if it was the other way around.

Blueberry’s fingers twitched as his boyfriend’s hands went to his pelvic region.

 

Red’s hands were making him feel really weird, like he had felt last night. The term was aroused if he remembered correctly, but feeling that way in front of his brother was embarrassing. The weirdest thing was that knowing his brother was watching, only made it worst.

 

“so how far did you and Red go?” his brother asked.

 

“Downstairs.” he answered confused, Red's hand did nothing to help.

 

Their hands were not intertwined anymore, Berry’s hands were gripping his pants as Red was now inside his pants, rubbing at his blue clit.

 

“that’s not exactly what he means blue. we only went as far as to holding hands, so don’t worry swap.” Red lied.

 

Blueberry wasn’t thrilled at the idea of lying to his brother, but he got the clue, and that also meant that what was happening under the table was a secret.

The fingers rubbing his clit got harsher, and he did his best to keep his blush and his sounds under control.

Swap didn’t seem to believe them, but to Blueberry's relief, kept silent.

A long silence had fallen over the kitchen, only breaking once in awhile when Fell put his mug down to turn the newspaper’s page.

Blueberry raised his hips a bit in order to find a more comfortable position.

The action caused Red’s fingers to accidentally slip inside of him, making him let out a loud moan.

 

Red jumped at the sound, looking in surprise at his brother who was having an extremely convenient coughing fit, he had almost forgot Fell was there.

He quickly pulled his fingers out while his brother was still coughing, any sound his lover could have made was muffled by the one’s of his brother.

There was a pause as they wait for Fell to finish.

 

“Are you alright” Blueberry asked, genuinely concerned.

 

“OF COURSE I AM, IT TAKES MORE THAN COFFEE TO KILL ME” Fell said, taking another sip.

 

“anyways, what did you wanna say bro? you opened your mouth but it’s not like we could have heard you.” Swap said.

 

“Y-yeah, I was s-saying th-that y-your sh-sh-shift is about to start!” Blue improvised quickly. Mentally thanking the gods that had decided to show mercy on him.

 

“yeah you’re right bro, welp i better be going.” Swap said. He stood up, patting his brother’s skull and prepared to leave.

 

An awkward silence filled the room until they heard the door close. Fell suddenly grabbed his brother by his jacket, choking him.

 

“I DON’T CARE IF YOU USE THE LITTLE CUNT AS YOUR CUM DUMPSTER, BUT IF YOU EVER DO SOMETHING LIKE THAT, OR DISTURB MY SLEEP AGAIN, I’LL LET ASHTRAY DUST YOU. HAVE I MADE MYSELF CLEAR?!”

 

“y-yes boss.” he answered as best as he could. His cervical vertebrae were hurting like a bitch.

 

His brother let him go and he fell back into his chair. Fell proceeded to leave the house, slamming the door behind him.

 

“he’s crazy.” Red whispered.

 

“You know, he cares a lot about you, more than he lets on. He fake coughed just to cover my moan so we didn’t get caught.” Blue said, taking a bite of his last taco.

 

“whaddya mean fake coughing?” he confusedly asked.

 

“When Papy fake coughs, his voice his higher than when he is doing it for real. Your brother is the same. Anyways, I have to go to work, see you later.” Blueberry said, kissing Red softly.

 

Blue turned around, but Red grabbed him by the arm before he could leave.

 

“why don’t you come see me at the station later?” he proposed with a wink.

 

“Okay?” Berry answered uncertain.

 

He left the house, slamming the door shut by accident. Red chuckled as he heard a loud “Sorry!” from outside.

 

***

 

Red started humming a random song, probably one from Napstaton’s show, as he took a swig of mustard. The station he was in was one of the many Swap had around the Underground, meaning that if he was desperate enough, he could always drink some honey.

He still thought that working here was total bullshit, but it’s not like he would tell Boss that. I mean, if it made him happy, then he would play sentry.

He stopped humming, as he heard footsteps coming toward him.

They were too fast to be Swap and too light to be his brother, so he could safely assume it was Blueberry. The shorter skeleton soon appeared, stopping in front of the station.

 

“So, why did you want to see me?” he asked, placing his hands on his hips.

 

“well, this morning, you sees to really enjoy yourself.” he grinned leaning forward.

 

“Well… y-yeah… I… I did...” said Blueberry, blushing a deep shade of crimson.

 

“no need to be ashamed babe, it’s normal to enjoy yourself. after all, a body like yours is all sensitive and virgin to this kind of attention, of course it would melt under my touch.”

Red smiled, watching Blue’s actions. He clearly wouldn’t need to do much in order to get what he wanted.

 

“would you like it if i played with your clit again? make you feel so good, that you can’t help but moan my name?” He whispered, smile growing as he saw Berry unconsciously move one of his hands closer to his crotch.

 

“Y-yes.” The other whispered back.

 

“and you love my fingers inside of you, but you feel like you needed something else, right?” He questioned.

 

“Yes.” Blue said, loud and clear this time.

The smaller was squirming in place, trying to ease the uncomfortable feeling between his legs.

 

“i can do anything you can think of, and even more but on two conditions. first, i want you to beg for it.” Everything was going just as Red planned.

 

“P-please Red.” Berry said, embarrassment audible in his every word.

 

“i think you can do better than a simple, please. beg for me again, but this time, do it while kneeling in front of me.” He teased.

 

The smaller moved, kneeling down between Red legs.

 

“Please Red.” He begged desperately, this embarrassing little game arousing him to no end without him understanding why.

 

“you’re almost there, but what exactly do you want?” He asked, caressing Blue’s cheek lovingly.

 

“Red, I need you to take me.” Blue whined.

 

“good, now for my second condition, you’re gonna please me first. you’re going to get under the station on your knees, and you’re gonna suck my cock.” he unzipped his pants as Berry moved. He pull his chair closer to Blue, pinning him in the small space, his face a inch from the organ.

 

“now, take it in that pretty mouth of yours, suck, lick, do anything you think could make me feel. go.” he half instruct.

 

For how much the other know and had experience, he wasn’t half bad. He was eager to please, making it his personal challenge to take in as much of him as he could, often choking on him, pulling back in get some air then try again. He smile as he watches the other finally taking him entirely in his mouth.

 

“sup” he heard Swap said on the other side of the station.

 

The yell of surprise was perfect to cover the loud moan as Blue sound of surprise was muffled by him.

 

“whatcha doing?” The taller ask.

 

“nothing” he tried to lied. His left hand when down to spell  “stop” in sign language, hoping the other was good enough to understand.

 

Blue watch curiously at Red hand, was this a joke or an acronym he didn’t know about, why was the other signing Emoq at him?

 

“there must be something since you were looking down and smiling”insisted Swap.

 

He avoids his best to look down at the little skeleton who was eagerly working up and down his shaft.

 

“I-i was just looking through my photos.” he tried, putting one of his hands on top of Blueberry’s head.

 

The smaller took this as an invitation to go faster, bringing his lover closer to climax and the taller one smile darkly.

 

“The funny thing is that…” he searching his hoodie pocket, finding and showing a black old flip-flop phone. He open it, revealing a photo of Red and Fell when they were younger.”you left it on the couch yesterday night, now i would have usually let it go but you’ve been acting weirdly since this morning, mind just telling me what’s going on?” The taller ask.

 

Red didn’t answer, afraid that he would let out a moan as he came inside Blue mouth. Swap quickly walk around the station, pulling Red’s chair. Blueberry lost balance, falling backwards, coughing on the thick liquid in his mouth. He made eyes contact with his brother, he look away in shame. He felt a strong grip on his arm, raising him on his feet.

 

“We are going home right now” he said.

 

He grab Red at the same time and teleported them at home. Fell look surprised when the three of them arrived, his expression quickly went to frustration when he saw Blueberry’s face.

 

“REALLY SANS, I LET YOU A CHANCE THIS MORNING AND YOU FUCK IT UP THREE HOURS LATER” He really didn’t sound impressed.

 

“This morning? mind to explain?” He glared at Red.

 

“I might have touched him under the table while we ate this morning” he half explain. He hears boss order Blueberry to get washed, which he did quickly. Red wonder if people in this universe weren’t supposed to be this open with their sexual life. Blue came back, hold Red hands and smile softly at him, trying to reassure the darker.

 

“Brother, I understand that you are upset, but please calm down” Berry try to appease.

 

His brother sigh, he places his hand on Berry’s and kneel down to his height.

 

“Look, sans, maybe it’s different in red’s world, but it’s not the kind of thing you do in public.” He said, Blueberry was surprised that he use his name, a thing he didn’t do since their twin version appear.

 

“NOT LIKE YOU HAVEN’T DONE IT BEFORE”they hear Fell comment but Swap ignored him.

 

“But Papy, I don’t see what’s wrong with it.” He truly didn’t understand, what was wrong with doing something that obviously please Red.

 

“you don’t understand, you're just…” He stop himself when he saw his smaller brother’s face.

 

“I am not just a kid, brother, why can’t you understand!” Blue almost yelled.

 

“look, i won’t treat you like a kid if it makes you happy but it’s just...right now it look more like lust than love to me” Swap voice was full of concern.

 

Red was confused, wasn’t love just a fancy word for lust? He looks at Blue who seems about to cry, he pulls him in his arms and rub small circles on his lover back. He glared at Swap, who seems really surprised.

 

“I really love Red” he heard Blue said between sobs.

 

He put his arms around his Blueberry’s head, hoping that the large hoodie should interfere with his hearing.

 

“why don’t you mind your how fucking business” he yells at Swap.

 

The other look take back at his outburst, even his brother seems surprised, but he seems to enjoy the show.

 

“‘cause you are going out with my brother, i know how ‘love’ work for you” Swap replied.

 

He couldn’t deny how twisted his love life had been before, sleeping around most of the time, sometimes that he couldn’t even remember. The only thing is that there were two ways for Swap to know about this, first, that Boss talk to him about this, which was unlikely or…

 

“you are just the same, that’s why don’t trust me with Blueberry”he realised  “but i’m not you, don’t put your fucking issue on me”

 

Swap look away in shame, he feels Blueberry move against him, but he keeps him close.

 

“i guess you are right… just, be careful careful with my bro’s emotions, he....’’ he stops himself.

 

Red was curious, but by the face of the other, he knew he wouldn’t finish the sentence. He pulls away from Blueberry, who look at him curiously.

 

“Is everything alright?” His lover asks.

 

He nods, looking at Swap who was walking to his room. Fell look like he lost any interest toward what was happening, walking back into the kitchen. The couple sat on the couch, Blue places his hand on top of Red’s one.

 

“I love you” the smaller said.

  
“I love you too” even if he didn’t know how to love by Blue’s universe standards, he could always learn.

**Notes for the Chapter:**

> Man, english is hard, anyways, hope you liked it!

**Author's Note:**

> I'm sorry, I realise the other version was incomplete, I fix that, I'm sorry for the inconvenience.


End file.
